The tooltip for the "Skip Busy Agents" parameter in Queues ends with this:
WARNING: When using the settings that set the 'ringinuse=no' flag, there is a NEGATIVE side effect. An agent who transfers a queue call will remain unavailable by any queue until that call is terminated as the call still appears as 'inuse' to the queue UNLESS 'Agent Restrictions' is set to 'Extensions Only'.
All of the above can be removed in 15, as current versions of Asterisk 13 and 16 don't have this limitation, and 15 does not support earlier versions of Asterisk.